Age-associated alterations in immune function in humans and various rodent models have been well documented. Elderly subjects have been shown to be more susceptible to both viral andbacterial infections and are also believed to be more susceptible to cancer. A series of clinical studies have revealed that elderly subjects, in contrast to their younger counterparts, exhibit poor cellular and humoral immune responses to vaccines even in the presence of standard adjuvants. Our laboratory has developed a thymus dependent phosphocholine (PC) antigen, 6-(O-phosphocholine) hydroxyhexanoate (EPC), that provides cross protection against multiple serotypes of Streptococcus pneumoniae in a mouse model. The phosphocholine (PC) moiety of S. pneumoniae, is located on the cell wall C-polysaccharide of all serotypes of S. pneumoniae and anti-phosphocholine (PC) antibodies are known to provide protection against challenge with virulent S. pneumoniae. The ability of EPC conjugates to provide protection were evaluated in X-linked immunodeficient (Xid) mice. Xid mice fail to produce anti-PC antibodies even when immunized with S. pneumoniae. Consequently, Xid mice are extremely susceptible to infectionwith S. pneumoniae. Immunization of Xid mice with EPC conjugates is capable of rescuing PC specific B cells from the bone marrow. Analysis of PC-specific IgG hybridomas from Xid mice revealed that several V-D junctional variants of the VH1 gene segment rearranged to different D and JH gene segments were used. The majority of Xid anti-PC antibodies exhibit an Asp - Gly95H replacement at the V:D junction. These Gly 95H VH1 variants associate with kappa1C L-chains to produce anti-PC antibodies that: 1) have low relative affinity for PC; 2) are heterocliticfor nitrophenylphosphocholine; and 3) fail to bind to or provide protection against S. pneumoniae. Single prototypic V-D variants of the T15-id(Asp95H), M603-id (Asn 95H) and M167-id (Asp 95H - Ala 96H) were also induced in Xid mice. The M603- like and M167-like antibodies bound to S. pneumoniae and provided protection against infection by S. pneumoniapneumoniae even though they exhibited Kas for PC which were lower than T15-id+ antibodies. These data demonstrated that small changes in the V-D junctional sequence of the T15 (VH1)heavy chain alter L-chain usage and the structure of the PC binding-site. In parallel studies,transgenic mice expressing anti while controlling the potential negative effects of retaining B cells that express autoreactive sIg receptors-PC specific IgH/L chain genes provided protection against challenge with virulent S. pneumoniae. Following immunization with EPC-KLH, these mice produced only IgM anti- PC antibodies of a single idiotype. In some cases, the level of protection is almost as good as that seen in normal mice which can produce both IgM and IgG anti-PC antibodies following immunization with EPC-KLH. Very little preimmune protection is seen in most of these transgenic mice, although, some IgH chain transgenic mice have a higher ID50 than normal controls prior to immunization. This suggests that the level of anti-PC antibodies in the serum is more important than the number of B cells present that are capable of producing anti-PC antibodies. PC-specific B cells appear to be autoreactive in that they fail to develop into mature B cells in Rag-/- mice. However, the anti- PC antibodies produced by these B cells are not associated with any pathology in situ. The antibodies can inhibit in vitro mast cellmigration induced by platelet activating factor (PAF), a PC- containing chemoattractant, however,these antibodies do not appear to alter in situ PAF induced monocyte and granulocyteinfiltration. This new PC-protein conjugate vaccine has been shown to provide complete or partial protection in mice challenged with every serotype of S. pneumoniae that has been able to lethally infect mice. The inclusion of EPC conjugates in a vaccine for S. pneumoniae could provide an inexpensive way of effectively immunizing elderly individuals against this pathogen which infects millions of people each year. - pneumoniae;vaccine;phosphocholine;platelet activating factor (PAF; anti-phosphocholine (PC) antibodies;Streptococcus pneumoniae